This invention relates to digital signal processing and more particularly to arrangements for reducing coder noise and bit rate.
A digital signal is typically defined as a signal which may exist as either of two possible discrete values or states. The value or state of a digital signal may physically correspond to, for example, the presence or absence of a fixed voltage. It is usually more convenient, however, to omit reference to physical conditions and to describe the value or state of a digital signal as being equal to either 0 or 1. The numbers 0 and 1 are called binary digits or bits.
A digital communication or storage system is designed to convey or store information that is expressed in the form of digital signals. In order to convey or store an analog signal such as speech, it is necessary to convert the analog signal into digital form. Analog to digital conversion may be accomplished by a device known categorically as a coder. Digital signals are converted back into a replica of the original analog signal by a decoder.
A basic technique for converting analog signals into digital signals is known as pulse code modulation (PCM). In a PCM coder, an analog signal is sampled at a succession of time intervals. The sampling rate, that is, the number of samples taken in a given amount of time, must be equal to or greater than twice the maximum significant frequency of the analog signal. Each sample is coded, that is, assigned a digital representation according to its amplitude. The digital representation of a sample, called a binary coded word, comprises a pattern of bits. The number of bits in a word determines the number of distinct or different words available for coding the sample amplitudes. If words are defined as having one bit, for example, there are two possible distinct words: 0 and 1. If words have two bits, there are four possible words: 00, 01, 10 and 11. There may be, for example, 8 to 13 bits per word in an actual PCM representation.
The amplitude of a sample, on the other hand, may assume any one of a virtually infinite number of values. Since it would be impractical to assign a distinct binary coded word for every possible value of the sample amplitude, a process known as quantization is necessary. In quantization, the range of expected sample amplitudes is divided up into a finite number of discrete amplitude values. Each discrete amplitude value corresponds to a different binary coded word, the total number of discrete amplitude values being equal to the number of possible binary coded words. For a given sample amplitude, the nearest discrete amplitude value is selected. The binary coded word which corresponds to the selected discrete amplitude value is the digital representation of the sample amplitude. A sampled analog signal is thereby said to be quantized and coded.
Since the quantization process only approximately represents the original analog signal, distortion or coder noise is introduced. The noise may be reduced by increasing the number of bits used to quantize each sample. Increasing the number of bits per sample also increases the bit rate of the coder, the coder bit rate being defined as the product of the sampling rate and the number of bits per sample. An increased coder bit rate, however, commands an increased share of the available capacity of a digital communication system.
Reduced coder noise and bit rate can be achieved with a variety of more complex quantization techniques. The techniques generally exploit the fact that adjacent samples of speech are very similar to one another. Adaptive data modulation (ADM) and adaptive differential pulse code modulation (ADPCM), for example, are well known refinements of the basic PCM scheme of sampling, quantization and coding.
It is an object of the invention to provide reduced coder bit rate at a given level of noise or conversely to provide reduced coder noise at a given bit rate. It is another object to provide an adaptive sampling rate coder whereby the average bit rate of the coder is reduced.